1. Field of the Invention
The present invention relates to steplessly variable transmissions and more particularly to a speed ratio control for steplessly variable transmissions. More specifically, the present invention pertains to steplessly variable transmissions provided with electronic control means for controlling the speed ratio in accordance with engine operating conditions. Further, the present invention also relates to a method for controlling a steplessly variable vehicle transmission.
2. Description of the Prior Art
In vehicles provided with steplessly variable transmissions, the speed ratio of the transmission is controlled in accordance with vehicle or engine operating conditions, such as the vehicle speed, the engine speed and the engine throttle valve position or the like. One example of such speed ratio control is shown in Japanese laid-open patent application No. 55-76709. In a conventional speed ratio control, the speed ratio is determined so that a desired engine speed is obtained with respect to a given engine throttle valve position. In other words, under a given throttle valve opening, a predetermined engine speed is obtained so that the engine produces a predetermined output.
In the usual operation of a vehicle, an acceleration is performed by actuating an engine control foot pedal to thereby increase the throttle valve opening. In this instance, it is desirable for obtaining a feeling of vehicle acceleration to have a rate of acceleration to be continuously maintained in an amount corresponding to the amount of actuation of the foot pedal. It should however be noted that, in a vehicle provided with the aforementioned steplessly variable transmission, wherein the speed ratio is controlled so that a predetermined engine speed is attained under a given throttle valve opening, the engine speed is immediately increased to the predetermined value as soon as the engine control foot pedal is depressed, and thereafter the engine speed and therefore the engine output is maintained during the acceleration. Since the drag force on the vehicle gradually increases as the vehicle speed increases, the driving effort applied to the vehicle is gradually decreased as far as the engine output is maintained substantially constant as described above irrespective of the increase in the drag force, with the result that the acceleration rate is correspondingly decreased. This will give the operator an unsatisfactory feeling of acceleration. A further disadvantage in the conventional control is that the engine speed is maintained substantially constant in acceleration and deceleration, and there is no dynamic change in the engine noise so that the operator will be dissatisfied in respect of the acoustic effect.